The invention relates to an electrolysis process and electrolytes therefor for producing an alkali metal.
Alkali metals are highly reactive elements and are not found in elemental form in nature. Typical reducing agents, such as hydrogen, are not strong enough to reduce the alkali metals from their compounds to the metallic state. Electrolytic reduction is necessary and was used historically in the classic experiments leading to the discovery of the alkali metals in elemental form in 1807 by Sir Humphry Davy, Assistant to Count Rumford/Thompson at the Royal Institution in London. Electrolytic reduction is used for industrial production of the alkali metals. The currently used process, on a worldwide basis, is the so-called xe2x80x9cDownsxe2x80x9d Process, which was introduced in the early part of the 20th century for the production of sodium and lithium from their chlorides.
The Downs Process uses a molten salt electrolyte consisting of a mixture of NaCl, CaCl2 and BaCl2 in order to reduce the melting temperature of the electrolyte to slightly below 600xc2x0 C. This makes the process more practical compared to using pure NaCl which has a much higher melting point of about 800xc2x0 C. Nevertheless, operating an electrolytic process at such temperature is difficult and presents numerous operating constraints. Because of the high operating temperature of the Down Process, the cell design uses concentrically cylindrical cathodes, wire mesh diaphragms, and anodes rather than the much more space efficient stacked multiple flat electrode and diaphragm element configuration that is normally used in electrochemical engineering practice. Furthermore, the high operating temperature would make a flat wire-mesh steel diaphragm so soft that it would be mechanically unstable and flap back and forth between anode and cathode causing partial shorting/arcing and thereby causing holes to be burned in the diaphragm. Holes in the diaphragm would allow back mixing of sodium produced at the cathode and chlorine produced at the anode, thereby causing low current efficiency of the cell. On the other hand, the concentric cylindrical configuration of the steel diaphragm between the electrodes avoids this difficulty because a wire-mesh cylinder is mechanically much stiffer and mechanically more stable than a flat wire-mesh screen of the same kind.
The above-described concentric cylindrical cell design of the Downs Process, necessitated by the high operating temperature of about 600xc2x0 C., also means that the Downs cell has very poor space efficiency. This translates directly into high capital and operating cost per unit production.
The high operating temperature of the Downs cell in combination with the fact that the molten mixed salt electrolyte has a freezing temperature only about 20xc2x0 C. below the cell operating temperature makes smooth operation of the cells difficult. Cell xe2x80x98freeze-upsxe2x80x99 and other xe2x80x9cupsetsxe2x80x9d are frequent and result in unusually high operating labor requirements for an industrial electrolytic process. This in turn is also the reason why the Downs Process is not amenable to automation. Lithium is currently produced by a modification of the Downs process.
Although there is a low temperature electrolytic process that deposits metallic sodium at the cathode from an NaCl/H2O solution, the sodium metal is not pure sodium but a liquid mercury/sodium amalgam containing a low percentage of sodium, usually about 0.5% Na. The balance of over 99% is mercury metal. This process is used to produce aqueous sodium hydroxide solutions by reacting the dilute sodium amalgam with water. See generally Sodium, Its Manufacture, Properties and Uses, by Marshall Sitting, American Chemical Society Monograph Series, Reinhold Publishing Corp., New York (1956) and Electrochemical Engineering, by C. L. Mantell, McGraw-Hill Book Co., Inc., New York, Toronto, London (1960). This process cannot be used to produce metallic sodium economically because of the problems and the cost of separating mercury from sodium. For example, separation by distillation is impractical because mercury has a much lower boiling point (357xc2x0 C.) than sodium (880xc2x0 C.) and it would be too costly to vaporize 99% mercury to obtain about 1% sodium as the residue.
In recent years fundamental physico-chemical studies have been carried out on electrolytes based on non-aqueous, organic solvents for alkali metal chlorides for battery applications. See J. Electrochem. Soc. Vol. 143, No. 7, pages 2262-2266, July 1996. None of this work resulted in a process for alkali metal production.
Therefore, there is an increasing need to develop an electrolytic process that can be used to produce an alkali metal more economically. There is also a need to develop a process that can improve operability such as, for example, making automation possible.
According to the invention a low temperature electrolysis process is provided, which comprises carrying out the electrolysis in the presence of a co-electrolyte and an alkali metal halide. The co-electrolyte comprises (1) a nitrogen-containing compound and optionally a Group IIIA halide, a Group IB halide, a Group VIII halide, or combinations of two or more thereof; (2) a Group IIIA halide, a Group VB halide, or combinations of a Group IIIA halide and a Group VB halide; or (3) water.
Also according to the invention an electrolysis process is provided, which comprises carrying out the process using a cathode comprising (1) a liquid alkali metal; (2) a liquid low melting alloy of two or more metals selected from the group consisting of bismuth, lead, tin, antimony, indium, gallium, thallium, and cadmium; or (3) a conductive liquid solvated alkali metal.
Further according to the invention an electrolyte is provided. The electrolyte comprises an alkali metal halide and a co-electrolyte that comprises (1) a nitrogen-containing compound and optionally a Group IIIA halide, a Group IB halide, a Group VIII halide, or combinations of two or more thereof, (2) a Group IIIA halide, a Group VB halide, or combinations of a Group IIIA halide and a Group VB halide; or (3) water.